Aircraft airspeed measurements are required for aerodynamic control, providing indicated airspeeds for stall speeds, landing speeds, never-exceed speeds, and true airspeeds for navigation, weapons release ballistics and a variety of other uses. Typically, aircraft speed is measured by pitot-static systems which provide indicated airspeed, that is, the ram or dynamic pressure compared to the static pressure for the particular altitude. The indicated airspeed provides information to the pilot for aerodynamic speed control to set such speeds as landing speed, best glide speed, maneuvering speeds and other aerodynamic speeds.
Indicated airspeeds are dependent on air density and may be significantly different from true airspeed. True airspeed is required to determine the actual path of the aircraft, for example, navigation, intercepts, bomb delivery, etc. Prior art systems generally measure indicated airspeed and use an air data computer or other calculating means to determine true speed. Several problems arise with the existing airspeed devices. First, measurement accuracy is dependent on the alignment of the pitot head with the incoming airflow. At large yaw angles or at high angles of attack, significant errors are introduced. Additionally, the operating range of the pitot-static system is limited. At low air speeds, the dynamic-static pressure differences are too small for accurate readings. Typically below approximately 30 knots, pitot-static systems become unusable. A system is needed which can produce accurate results over a speed range from near zero to 250 Kts and which can accept freestream flow angle changes up to 30 degrees. The lower speed range is necessary for accurate weapons delivery and flight control for hovering or slow moving aircraft and the angle range is necessary to provide accuracy over the angle-of-attack and side-slip operating envelope of a typical combat aircraft.
Numerous devices have been developed to measure the low speed operation of aircraft. Typically, these devices calculate indicated airspeed using complex computations of main rotor longitudinal and lateral cyclic pitch positions, along with collective pitch and tail rotor pitch. These devices typically require additional input data, such as inertial acceleration, inertial velocity, and wind velocity. The complexity, cost and weight of these types of systems have prevented the wide-spread acceptance of these systems.
A simple, low cost system is needed which can provide accurate data over a broad range of airspeeds and directions.